1. Field of the Invention
The present invention is related to bicycle suspension systems, and more specifically to a simplified suspension employing an axially moveable seat-stay arrangement with chainstays operating as a spring/dampener.
2. Description of the Prior Art
There are many known bicycle frame designs which provide a degree of damped motion to the rear wheel, so that transmission of sudden dips, bumps, and terrain variations to the rider via the bicycle frame are reduced. These designs generally comprise a rigid main frame (the seat tube—top tube—head tube—down tube “diamond”) with a pivoting swing arm assembly comprising the seatstays and chainstays. In certain arrangements the pedal crank assembly forms a portion of the main frame, and in others is forms a portion of the swing arm. A spring and elastomeric or fluid dampener combination is the traditional mechanism for shock absorption, although solid elastomeric, purely-liquid, gas, and other arrangements have been used in place of the spring and dampener “shock absorber”. Some types of suspension systems provide the shock absorber arranged in the plane of the seatstays, often forming a coaxial leg of the seatstay “wishbone” or replacing one or more stays, while other types of suspension systems provide a shock absorber disposed within the main frame.
While true of many vehicles, light weight and simplicity of design are critical elements when selecting components for a bicycle. The majority of prior art rear-suspension bicycle frame arrangements have suffered from excessive weight, excessive complexity, or both. In order to reduce added parts, and hence weight and complexity, one class of rear suspension bicycle frames employ the beam formed by the chainstays and bottom bracket as the spring member of a shock absorber. The chainstays are rigidly attached, as opposed to pivotally attached, to the bottom bracket or other portions of the main frame. For this reason, we refer to such frame arrangements as a “pivotless” suspension. General examples of such pivotless suspension frames include U.S. Pat. No. 7,168,726 to Klein, U.S. Pat. No. 6,932,371 to Perez et al., U.S. Pat. No. 6,406,048 to Castelano, U.S. Pat. No. 6,109,637 to Kirk, U.S. Pat. No. 5,865,456 to Busby et al., and U.S. Pat. No. 5,797,613 to Busby.
One problem with current pivotless suspension frames is a need to provide enhanced rigidity during shock absorption. For example, there is a need to resist twist of the rear wheel about its radius as the wheel moves up and down during shock absorption. It is also important to resist motion along the axis of the of the wheel axle. And again, it is important to do so while minimizing weight and complexity.
Noteworthy is U.S. Pat. No. 7,168,726 to Klein, according to which a bushing is disposed between two portions of a seatstay wishbone. The bushing is rigidly attached to a first part (the wheel side) of the wishbone with a series of bolts. A linear, sliding extension bearing is rigidly attached to the bushing by the bolts. The sliding extension bearing is disposed inside the second part (the seat side) of the wishbone. The bushing serves as the shock absorber, and the sliding extension servers as a mechanism to provide the aforementioned rigidity. In this design, the chainstays also provide a certain degree of spring to the system, although the primary shock absorption is provided by the bushing.
The design disclosed in U.S. Pat. No. 7,168,726 suffers from several disadvantages. First, in any such system there will be a tendency of the sliding members to resist sliding, thus reducing the effectiveness of the shock absorption. This may be due to tightness of fit between components, dirt, rust, fluid or other material between the bearing and the inner wall of the tube in which it travels, pinching due to force applied off-axis to the angle of motion of the bearing (such as side deflection and torque), etc. From whichever source, the force resisting relative motion between the sliding members is referred to as stiction. The linear arrangement of the bearing according to U.S. Pat. No. 7,168,726 is very susceptible to stiction caused by of axis forces. Any such off-axis forces tend to cause the bearing to pinch inside the second part of the wishbone. U.S. Pat. No. 7,168,726 relies on flex of the bolt system and of the stays to compensate for the off-axis forces, not a function anticipated for in the design of these parts.
Second, there are a number of specialized parts required for the system disclosed in U.S. Pat. No. 7,168,726. Fabricating same for such an application means higher cost and more limited availability of such parts. Further, the structure of the bearing, comprised of parallel disks and a hollow connecting shaft, introduce several points of failure. For example, significant off-axis force can deform one or both disks, break the shaft and/or bolt holding the system together, deform or break the second wishbone part in which the bearing slides, etc. The high force per surface area of this arrangement increases the likelihood of failure. Exacerbating this problem is the fact that the design requires a hole be provided in, and perpendicular to the axis of, the bolt holding the assembly together. The mounting for the rear brake is introduced through this hole. This hole is a point of high stress during combined shock absorption and braking, thus another point of potential failure of the system. Any of these failure modes could introduce the dangerous condition of the rear wheel being inadequately supported by the stay arrangement.
Third, the system disclosed in U.S. Pat. No. 7,168,726 requires that the bushing be in contact with the first and second wishbone parts. This limits the extent of shock absorbing motion (travel) available to the rear wheel.
A second system of note is that disclosed by Castellano in U.S. Pat. No. 6,406,048. According to this system, a generally flat, vertically flexible, bifurcated chainstay serves as a spring, and a shock absorber in the seatstay arrangement absorbs shock and limits vertical deflection of the rear wheel. However, similar to U.S. Pat. No. 7,168,726, this system has a number of disadvantages. First, the shock-absorber system is comprised of planar parts designed for motion along the axis of the shock absorber. Any off-axis motion creates friction and binding of the parts forming the shock absorber. Second, in addition to the unique seatstay arrangement, the shock absorber requires numerous specialized parts. Such a shock absorber is expensive, heavy, and difficult to service. The many parts provide regions for dirt and grime accumulation. And as the number of elements increase, the chances of breakage or part failure increase. Third, the shock absorber is of the oil-filled type, requiring specialized maintenance skills, parts, tools, etc.
For these reasons and others, there is a need in the art for an improved pivotless rear suspension bicycle frame providing high reliability, low complexity, low cost, ease of installation and repair, high safety, and effective shock absorption. The present invention is designed to address each of the needs.